1. Field of the Invention
This invention relates to a zinc- or a zinc alloy-plated member having chemical conversion coatings which do not contain hexavalent chromium or fluorides, which are harmful to the human body and to the environment. It also relates to automotive parts, electronic or electrical equipment parts, or building construction parts, which are made from such a corrosion resistant member.
2. Description of the Related Art
Zinc plating and zinc alloy plating are widely used to prevent corrosion of steels and other metals. In this specification, zinc plating and zinc alloy plating will be collectively referred to as “zinc-based plating” and a zinc-plated member and a zinc alloy-plated member will be collectively referred to merely as a “zinc-plated member”.
A zinc-plated member exhibits corrosion resistance utilizing the sacrificial corrosion preventing ability (preferential dissolving) of zinc. Therefore, in an unprotected state, the corrosion resistance of zinc-based plating decreases in a relatively short period of time. Accordingly, in order to increase the corrosion resistance of zinc-based plating, chemical conversion treatment is usually carried out after plating to form a protective film atop the plating. Zinc phosphate treatment and chromate treatment are the chemical conversion treatment most commonly utilized for this purpose.
Zinc phosphate treatment is carried out under heating using an aqueous solution containing phosphoric acid ions and zinc ions. The solution also contains fluorine ions or fluoride complex ions as an etching agent. The zinc phosphate film obtained has surface irregularities, so it increases the adhesion of paint or other coating applied atop the film. A zinc phosphate film is used as primer coating for painting. When painting is not carried out, a zinc phosphate film gives the zinc-plated member a poor appearance, and the effect of the plating on improving corrosion resistance is small. Furthermore, the fluorine ions or fluoride complex ions as an etching agent are severely corrosive, so there are strict regulations concerning how it can be discarded.
Chromate treatment is carried out using an aqueous solution of chromic acid anhydride. This aqueous solution may further contain one or more suitable auxiliary components selected from various mineral acids (H2SO4, HNO3, HF, H2SiF6, H3PO4, HCl, and the like), colloidal silica, water soluble resins, and the like. The color of the resulting chromate film varies in accordance with the type of mineral acid which is added. In bright chromate treatment, the color of the film varies from colorless to light yellow. In colored chromate treatment, other colors, such as black or green, are possible.
Chromate treatment is generally classified as coating type treatment (in which washing is not carried out after treatment), reaction type treatment (in which washing is carried out after treatment), and electrolysis type treatment. In each case, the reduction of Cr6+ to Cr3+ (in coating type and reaction type treatment, the reduction is carried out by dissolving zinc in the plating surface, and in electrolysis type treatment, the reduction is carried out by electrolytic reduction) is the first step in film formation. The resulting chromate film has a complicated composition, but it is thought that the composition is approximately a chromium chromate (mCr2O3.nCrO3.xH2O). Namely, it is a mixture of hexavalent chromium and trivalent chromium, so it necessarily contains hexavalent chromium.
Another type of chromate film is a chromium phosphate type chromate film in which a mineral acid in the form of phosphoric acid is added to a chromic acid aqueous solution, and CrPO4 is introduced into the film. In this case, chromium chromate is also present, so the film contains hexavalent chromium.
The complex hexavalent chromium in the chromate film is soluble, and if the hexavalent chromium dissolves, it is reduced to trivalent chromium and film formation takes place, so it imparts self-repairing properties to the chromate film. The self-repairing properties give a chromate film excellent corrosion resistance.
However, as is well known, hexavalent chromium is harmful to humans. Chromate treatment employing hexavalent chromium entails expensive treatment of waste liquid. In addition, when a material having a zinc-based plating which has undergone chromate treatment is discarded, hexavalent chromium dissolves from the chromate film, and the hexavalent chromium can cause environmental problems. For these reasons, there has come to be a desire for a chemical conversion treatment liquid which does not contain hexavalent chromium.
Some known examples of such chemical conversion treatment liquids which are free of hexavalent chromium are those disclosed in Japanese Published Unexamined Patent Application Hei 7-126859, Japanese Published Unexamined Patent Application Hei 10-183364, U.S. Pat. No. 4,148,670, and Japanese Published Unexamined Patent Application Sho 52-131937.
While it is true that these chemical conversion treatment liquids do not contain hexavalent chromium, but except for that disclosed in Japanese Published Unexamined Patent Application Hei 10-183364, each of the treatment liquids contains fluorides. Fluorides are strongly corrosive, and they have a harmful effect on human bones, so discarding of fluorides is strictly controlled, making their use troublesome.
A film which is formed by a conventional chemical conversion treatment liquid which uses trivalent chromium and is free of hexavalent chromium cannot adequately form a film and has poor corrosion resistance compared to a chromate film formed by reduction of hexavalent chromium to trivalent chromium. On the other hand, when a conventional chromate film relying on hexavalent chromium is heated to a high temperature, the hexavalent chromium in the film becomes insoluble, so the self-repairing properties due to dissolving of hexavalent chromium decrease, and the corrosion resistance of the film when heated enormously decreases.